Dissipation of Magnetic Energy in Collisionless Accretion Flows

Abstract

In a series of investigations, we explore magnetic dissipation in collisionless accretion flows of black hole (BH) coronae. We study energy partition between electrons and protons via magnetic reconnection for the idealized geometry of antiparallel reconnection, and develop an electron heating prescription, motivated by the results of fully-kinetic particle-in-cell (PIC) simulations, which depends on plasma beta_i (the ratio of proton thermal pressure to magnetic pressure) and magnetization sigma_w (the ratio of magnetic energy density to enthalpy density). We extend the study of antiparallel reconnection to the more general case of 'guide field' reconnection, in which an out-of-plane component of magnetic field B_g is superposed on antiparallel reconnecting field lines (with magnitude B_0); we study the dependence of energy partition between electrons and ions on plasma parameters beta_i, sigma_w, the guide field strength b_g=B_g/B_0, the initial electron-to-ion temperature ratio T_e/T_i, and the ion-to-electron mass ratio, m_i/m_e; we consider mass ratios up to the realistic value, m_i/m_e=1836. We develop a general prescription for electron irreversible heating efficiency via magnetic reconnection as a function of beta_i, sigma_w, b_g, Te/T_i, and m_i/m_e, and we explore with guiding center formalism the mechanism of electron heating in guide field reconnection. We study the Kelvin-Helmholtz (KH) instability with linear stability analysis, magnetohydrodynamic simulations, and PIC simulations; for a test case, we explore magnetic dissipation and heating in KH-induced reconnection. We implement the Esirkepov method of current deposition, as a way of reducing numerical heating in our PIC experiments. This work addresses the crucial question of dissipation of magnetic energy in collisionless accretion flows of BH coronae, and shows that KH-induced magnetic reconnection may both heat particles and accelerate particles into a power-law energy tail.